1. Field of the Invention
This invention relates to vaccines for stimulating immune responses in human and other hosts, and, in particular, relates to recombinant viruses that express heterologous antigens of pathogenic viruses, such as Ebola, HIV, hepatitis, and influenza viruses.
2. Description of the Related Art
Current techniques for developing vaccines are largely based on the concept of using denatured virus or purified viral proteins made from bacteria. These types of vaccines may be effective for only a limited number of infectious agents, and the protection rates are limited.
For viruses that contain membrane (envelope) glycoproteins (GPs), including the Ebola virus and the HIV virus, use of denatured virus or purified viral proteins often does not work satisfactorily. There may be several reasons for this. First, the GPs of these viruses are sensitive to the denaturing procedures so that the epitopes of the proteins are altered by the denaturing process. Second, the sugar moieties of the GPs are important antigenic determinants for neutralizing antibodies. In comparison, proteins made in bacteria are not properly glycosylated and can fold into somewhat different structures that can have antigenicities different from those of the natural viral proteins. Further, many vaccines that are based on attenuated or denatured virus provide a weak immune response to poorly immunogenic antigens. In addition, the vaccine preparations frequently offer only limited protection, not life-long immunity as desired.
Other vaccine approaches express antigens by plasmids directly injected into the body, the so-called naked DNA or DNA vaccine technology. These methods involve the deliberate introduction of a DNA plasmid carrying an antigen-coding gene by transfecting cells with the plasmid in vivo. The plasmid expresses the antigen that causes an immune response. The immune response stimulated by DNA vaccine can be very inefficient, presumably due to low levels of uptake of the plasmid and low levels of antigen expression in the cells. DNA vaccines are also characterized by an extremely short antigen expression period due to vector degradation. In addition, DNA vaccines are difficult and costly to produce in large amounts.
Replication-competent, live vaccinia viruses have also been modified for expression of the genes for hepatitis B (HBV), human immunodeficiency virus (HIV), influenza and malaria antigens. In some instances, though, the immune response of recombinant vaccines is often of limited nature and magnitude. Thus, for example, while peripheral immunization with vaccinia influenza recombinants provides good protection against lower respiratory tract infections, it fails to induce immunity in the upper respiratory tract. On the other hand, peripheral immunization with recombinant vaccines may prove ineffective when local rather than systemic immunity is required, as in, for example, the gastrointestinal tract.
Vaccination with recombinant vaccinia virus expressing Ebola virus GP has been attempted to confer partial protection in guinea pigs. Gilligan, K. J., et al., Vaccines, 97:87-92 (1997). Vaccination with DNA constructs expressing either GP or nucleocapsid protein (NP) protects mice from lethal challenge with Ebola virus. Vanderzanden, L., et al., Virology, 246(1):134-44 (1998). However, each of these approaches has its own set of limitations that make them less then ideal choices for Ebola virus vaccines in humans. For example, vaccinia virus rapidly kills vector-infected cells. Consequently, the vaccine antigen is expressed for only a short time. However, the major limitation for this type of approaches is that the replication of vaccina virus causes the immune system to react mainly to the vaccinia proteins, only small portion of the immune responses is targeted to the antigen of the pathogenic virus. This phenomenon has been termed “antigen dilution”.
Previous attempts to remedy these deficiencies, including expression of vaccine antigens through viruses having stronger promoters, such as poxvirus, have not met with significant success.
As yet, no vaccine has been effective in conferring protection against HIV infection. Attempts to develop vaccines have thus far failed. Certain antibodies reactive with HIV, notably anti-GP160/120 are present at high levels throughout both the asymptomatic and symptomatic phases of the HIV infection, suggesting that rather than playing a protective role, such antibodies may in fact promote the attachment and penetration of the virus into the host cell. More significantly, current vaccines do not induce efficient cellular responses against the infected cells, the source of newly released virions.